This invention relates to magneto-optical recording media of the type wherein information is recorded and reproduced with the use of heat and light of a laser beam.
For optical recording media of magneto-optical memory type, there are well known a number of materials for a recording layer thereof, for example, MnBi, MnAlGe, MnSb, MnCuBi, GdFe, TbFe, GdCo, PtCo, TbCo, TbFeCo, GdFeCo, TbFeO.sub.3, GdIG (gadolinium iron garnet), GdTbFe, GdTbFeCoBi, CoFe.sub.2 O.sub.4, etc. These materials are deposited on transparent substrates of plastic material or glass as a thin film by any suitable thin-film forming techniques such as vacuum deposition or sputtering. The features common to these magneto-optical recording thin film layers are that the axis of easy magnetization is perpendicular to the film surface and that Kerr and Farady effects are great.
By taking advantage of these features, the following optical magnetic recording process is contemplated for such optical magnetic recording thin films. At the outset, the entire film is rendered "0", that is, uniformly magnetized or erased. A laser beam is applied to the film at the site where it is desired to record "1". The temperature of that region of the film exposed to the beam is increased, and coercive force Hc approaches 0 when the temperature approaches and then exceeds the Curie point. When the temperature is allowed to return to room temperature after extinction of the laser beam, the magnetization is reversed by the energy of demagnetizing field. Alternatively, the temperature is allowed to return to room temperature with an external magnetic field being applied in an opposite direction to that at the initial during exposure to the laser beam, and then magnetization is reversed. There is recorded a signal "1". The remaining portion of the film where no laser beam is incident remains "0" because the initial state is "0".
The recorded data in the optical magnetic memory is read out by similarly using a laser beam to detect the magneto-optical effect, that is, the rotation of the plane of polarization of reflected light with respect to the incident laser beam due to the reversal of magnetization.
Requirements imposed on such optical magnetic recording media are:
(1) that the Curie point is of the order of 100.degree. to 200.degree. C. and the compensation point is close to room temperature, PA0 (2) that noise-inducing defects such as grain boundary is relatively small, and PA0 (3) that a magnetically and mechanically uniform film is attained over a relatively large area without resorting to a high temperature or long term film formation process.
In the light of these requirements, a great attention is recently drawn to amorphous perpendicular magnetizable thin films of rare earth element-transition metal among the above-mentioned materials. Optical magnetic recording media having such amorphous perpendicular magnetizable thin films of rare earth element-transition metal, however, have a storage problem. If the magnetic thin film layers are stored in contact with the ambient atmosphere, rare earth elements therein are preferentially erroded or oxidized by oxygen and moisture in the atmosphere, losing the necessary information recording and reproducing ability.
For this reason, most efforts are generally directed to those recording media of the construction having a protective layer interposed between the substrate and the magnetic thin film layer. Known protective layers are vacuum deposited films of inorganic materials such as SiO and SiO.sub.2 as well as coatings of room temperature curable resins. However, these protective layers provide insufficient moisture protection, leaving a problem of shelf deterioration.
One approach to this problem is our proposal to form a plasma-polymerized film on the substrate (see Japanese Patent Application No. 60-180729 filed Aug. 17, 1985).
We have also proposed to form a plasma-polymerized film on a plasma-treated substrate and to form a magnetic thin film layer thereon in order to achieve an improvement in the adherence between the substrate and the magnetic thin film as well as improvements in the performance and durability of the medium (see Japanese Patent Application No. 60-181327 filed Aug. 19, 1985).
A still further proposal is to form a plasma-polymerized film on the substrate of a magnetic thin film layer in order to prevent deterioration of the magnetic layer by moisture or other undesirable substances from its surface (see Japanese Patent Application No. 60-182767 filed Aug. 20, 1985).
These proposals, however, do not fully meet the required properties of magneto-optical recording media. There is a need for magneto-optical recording media having more improved properties.